Under the terms of the deal, Iran has agreed to halt enrichment of uranium above 5% purity, and "neutralise" its stockpile of near-20%-enriched uranium.

In return, the world powers agreed to suspend certain sanctions on trade in gold and precious metals, Iran's automotive sector, and its petrochemical exports.

Senior US administration officials told the BBC that some of the sticking points that had been resolved in the past six weeks centred on how often inspectors would be allowed to visit Iran's nuclear sites. Officials say as a result of the deal:

From 20 January, Iran will start diluting its stockpile of 20%-enriched uranium

All 20%-enriched uranium will be gone within six months

Daily access will be provided to the Fordo uranium enrichment site near the holy city of Qom

Monthly inspections will be allowed at the Arak heavy water reactor

Iran's nuclear sites

In his statement, President Obama said: "Beginning 20 January, Iran will for the first time start eliminating its stockpile of higher levels of enriched uranium and dismantling some of the infrastructure that makes such enrichment possible."

In return, he added, over the next six months the US and the other five powers would begin to implement "modest relief" so long as Iran fulfilled its obligations.

"Meanwhile, we will continue to vigorously enforce the broader sanctions regime, and if Iran fails to meet its commitments we will move to increase our sanctions," he said.

And in a warning to his critics in the US Congress, who want to impose additional sanctions, he said he would veto any legislation enacting new sanctions during the negotiations.

Iran's senior nuclear negotiator, Abbas Araqchi, told Iranian TV that under the deal Iran would not limit its work on research and development, but it would halt the increase in facilities.

"For example, no new centrifuges will be added, no new centrifuges will be produced, and no new facilities will be installed at Arak. We will not expand the quantity of our programme, but we will expand the quality," he said.

He also said that the international negotiators had agreed to transfer $4.2bn (£2.6bn) of oil revenues in eight stages over six months.

UK Foreign Secretary William Hague welcomed "an important step towards peacefully resolving the Iranian nuclear issue, on which comprehensive negotiations will now start".

US Secretary of State John Kerry said that while implementation of the agreement was a critical, significant step, it was the next phase - of negotiating a comprehensive deal - that posed a "far greater challenge", to guarantee Iran's nuclear programme was peaceful.

"The negotiations will be very difficult but they are the best chance that we have to be able to resolve this critical national security issue peacefully and durably."